Method for fluid pressure control in a closed system

ABSTRACT

A method for controlling a system pressure within a closed system includes sending a signal to a pressure control valve corresponding to a pressure set point and actuating the pressure control valve to vary a pilot pressure of a control fluid contained within a pressure control line that is fluidly connected to a pressure regulator. A diaphragm of the pressure regulator is disposed between the pressure control line and a system line and acts on a fluid with the system line to modify the system pressure.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of U.S. National Stage application Ser.No. 15/306,917 filed Oct. 26, 2016 for “METHOD FOR FLUID PRESSURECONTROL IN A CLOSED SYSTEM”, which in turn claims the benefit of PCTInternational Application No. PCT/US2015/027955 filed Apr. 28, 2015 for“METHOD FOR FLUID PRESSURE CONTROL IN A CLOSED SYSTEM”, which in turnclaims the benefit of U.S. Provisional Application No. 61/987,250 filedMay 1, 2014 for “METHOD FOR FLUID PRESSURE CONTROL IN A CLOSED SYSTEM”by P. N. Dufault and T. A. Anderson.

BACKGROUND

The present invention relates generally to controlling one or moresystem parameters and, more particularly, to fluid pressure controlwithin a closed system.

Industrial systems that control various system parameters (e.g.pressure, flow rate, temperature, and the like) often encounter varioussystem disturbances. In order to maintain the system within establishedparameters, the control scheme for the system is designed to respond toenvironmental changes and variable properties of fluids or materialscontained within the system. Such control systems often detect andcounteract gradual changes in the system through monitoring parameterscritical to system performance.

Some industrial systems utilize sprayers to dispense material (e.g.paint, adhesive, epoxy, and the like) at a specific pressure and flowrate. In some systems that operate continuously or for relatively longperiods of time at a single pressure and flow rate, the pressure andflow rate reach steady state. Thus, minor changes in the material and/orsystem performance can be carefully monitored and counteracted by aconventional control scheme.

However, when such systems operate at multiple pressure and flow ratecombinations in which some conditions operate for relatively shortdurations, the pressure and flow rate do not reach steady state.Pressure and flow rate changes and/or fluctuations during thesetransient periods within the system are problematic for control systemsbecause conditions are different at the sprayer outlet than atmeasurement locations within the system. Failing to account for thesetransient conditions can result in over-dispensing or under-dispensingmaterial.

In some traditional control schemes, transient periods are controlled bysegregating system operating conditions and performing a calibrationroutine prior to performing each operation. However, calibrationroutines increase manufacturing costs and disrupt manufacturing workflow because production pauses during the calibration routine. In othertraditional control schemes, transient periods are controlled bydispensing excess material until the system reaches steady state. Oncethe system is at steady state, the traditional control scheme is capableof accounting for minor disturbances. However, dispensing excessmaterial increases material costs.

Therefore, a need exists for controlling the pressure and flow rate ofan industrial system that can cost-effectively adapt to multipleoperating conditions, environmental changes, and transient conditions.

SUMMARY

A method for controlling a system pressure within a closed systemincludes sending a signal to a pressure control valve corresponding to apressure set point and actuating the pressure control valve to vary apilot pressure of a control fluid contained within a pressure controlline that is fluidly connected to a pressure regulator. A diaphragm ofthe pressure regulator is disposed between the pressure control line anda system line and acts on a fluid with the system line to modify thesystem pressure.

A method of varying a system pressure of a sprayer system includesactuating a spray gun to stop a flow through the sprayer system, using acontroller to establish a pressure set point, sending a signal from thecontroller to a pressure control valve corresponding to the pressure setpoint, and actuating the pressure control valve to vary a pilot pressureof a control fluid within a control line that is fluidly connected to apressure regulator. A diaphragm of the pressure regulator fluidlyseparates the control fluid from a fluid contained within a system lineand acts on the fluid to vary the system pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an industrial sprayer system.

FIG. 2 is a flow chart showing a method for controlling a pressure ofthe industrial sprayer system in FIG. 1.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram of industrial system 10 for dispensingmixed material 12 from sprayer 14, such as a passive proportionersystem. Industrial system 10 includes, among other components describedhereafter, material supply systems 16 and 18, which contain materialcomponents 20 and 22, respectively. Material supply system 16 is fluidlyconnected to meter 24 with supply line 26, and material supply system 18is fluidly connected to meter 28 with supply line 30. Material supplysystem 16 acts on material component 20 to increase its pressure frominitial pressure P0 to supply pressure P1. Similarly, material supplysystem 18 acts on material component 22 to increase its pressure frominitial pressure P0 to supply pressure P2. Material supply systems 16and 18 can be pressurized tanks containing material components 20 and22, respectively. Alternatively, material supply systems 16 and 18 caninclude feed pumps or other circulating components that act on materialcomponents 20 and 22, respectively. As such, initial pressure P0 canrange from ambient pressure (0 kPa gage) to a pressure suitable forsupplying material components 20 and 22, typically no greater than 2068kPa gage (300 psig). Additionally, initial pressure P0 for materialsupply system 16 does not necessarily equal initial pressure P0 formaterial supply system 18. For instance, initial pressures P0 can betailored to the material properties of material components 20 and 22.Meters 24 and 28 are disposed along supply lines 26 and 30,respectively. Supply lines 26 and 30 fluidly connect material supplysystems 16 and 18, respectively, to mixed material line 32 at junction38 where supply lines 26 and 30 join. Mixed material line 32 fluidlyconnects supply lines 26 and 30 at junction 38 to spray gun 14. Meters24 and 28 are arranged in parallel and cooperate to supply materialcomponents 20 and 22 to mixed material line 32 where components 20 and22 combine to form mixed material 12 having mixed pressure Pmix. Meters24 and 28 supply mixed material 12 to sprayer 14 at flow rate R where itis selectively dispensed.

Pressure regulator 40 is disposed along mixed material line 32 to reducemixed pressure Pmix to system pressure Ps prior to dispensing mixedmaterial 12 from spray gun 14. Adjustment of system pressure Ps isaccomplished by using control valve 42 to vary pilot pressure Pp.Control valve 42 is disposed along control pressure line 44, whichcontains control fluid 46 and extends from control fluid source 47 topressure regulator 40. Control fluid 46 acts on diaphragm 48 of pressureregulator 40 to modify system pressure Ps when system 10 is in a closedstate. An increase in pilot pressure Pp increases system pressure Ps dueto force application of diaphragm 48 on mixed material 12. A decrease ofpilot pressure Pp decreases system pressure Ps due to a force reductionfrom diaphragm 48 on mixed material 12. When diaphragm 48 reduces forceapplied to mixed material 12, it acts on control fluid 46. Pilotpressure Pp of control fluid 46 is maintained by allowing a portion ofcontrol fluid 46 to return to control fluid source 47. In someembodiments, pressure regulator 40 is an air-operated, low flow pressureregulator.

System pressure Ps and flow rate R are managed by controller 50.Pressure transducer 52 disposed downstream from pressure regulator 40produces signal 51, which is a voltage or current of pressure transducer52. Signal line 54 electrically connects pressure transducer 52 tocontrol valve 42, and signal line 56 electrically connects control valve42 to controller 50, each signal line transmitting signal 51 tocontroller 50. Signal lines 57 and 58 electrically connect flow ratesensors 60 and 62 to controller 50, respectively. Flow rate sensor 60detects flow rate R1 flowing through meter 24, and flow rate sensor 62detects flow rate R2 flowing through meter 28. Flow rates R1 and R2 aretransmitted to controller 50 in the form of signals S2 and S3,respectively, which like signal 51, are voltage or currents from sensors60 and 62, respectively. Based on values of signals 51, S2, and S3,controller 50 executes a controlling scheme to modify flow rates R1 andR2 flowing through meters 24 and 28, respectively, and to modify systempressure Ps by commanding control valve 42 to change pilot pressure Pp.Material component 20, flowing at flow rate R1, combines with materialcomponent 22, flowing at flow rate R2, within mixed material line 32 toproduce mixed material 12, flowing at flow rate R. Controller 50modifies pilot pressure Pp by sending control signal C1 to control valve42 with control line 64 and modifies flow rates R2 and R3 by sendingcontrol signals C2 and C3 to meters 24 and 28 with control lines 66 and68, respectively.

Transient conditions exist within system 10 when actuating spray gun 14to close system 10, which is typically accomplished with an air-actuatedsolenoid valve (not shown in FIG. 1) or a trigger of spray gun 14 (notshown in FIG. 1). Because flow rates are measured at meters 24 and 28and not at spray gun 14, changes of system pressure Ps and flow rate Rlag changes to pilot pressure Pp and flow rates R1 and R2. If controller50 causes pressure regulator 40 to maintain a constant system pressurePs when system 10 is closed, then the pressure at spray gun 14 increasesdue to the lack of flow-based pressure drop within system 10.Subsequently, when system 10 is opened (i.e. from opening the solenoidvalve or trigger within spray gun 14), a burst of flow, driven by theprior pressure increase, causes non-uniform application of mixedmaterial 12. If controller 50 causes pressure regulator 40 to increasesystem pressure Ps while system 10 is closed, then effects from a burstflow are amplified. When controller 50 causes system pressure Ps todecrease while system 10 is closed, hysteresis effects increase theerror between the target pressure and system pressure Ps. The resultingsystem pressure Ps will not dispense mixed material 12 from spray gun 14at the desired flow rate R.

Moreover, material property and/or environmental changes impact systempressure Ps and flow rate R during operation. For example, materialcomponents 20 and 22, respectively, are periodically replenished.Because newly added material components 20 and 22 can have differenttemperatures from each other and from the previously dispensedmaterials, properties such as viscosity can affect flow rate R assupplied to sprayer 14. Additionally, mixed material 12 can partiallycure within mixed material line 32 and, over time, foul mixed materialline 32. As such, mixed material line 32 is periodically cleaned withsolvents. Environmental changes such as ambient temperature and humiditychanges also affect the properties of material components 20 and 22.However, system 10 is designed to operate over a range of systempressures Ps and a range of flow rates R, each operating conditionhaving duration.

Some spraying applications involve several discrete operatingconditions. For example, three operating conditions could be used insequential order: 1) dispense 100 cc/min at 68.9 kPA (about 10 psi) for10 seconds, 2) dispense 200 cc/min at 137.9 kPa (about 20 psi) for 15seconds, and 3) dispense 50 cc/min at 34.5 (about 5 psi) for 2 seconds.Without the aid of method 70 described below, the transient conditionsof system 10 are counteracted by performing repeated calibrationprocedures and/or by discharging mixed material 12 between operatingpoints until steady state conditions are present within system 10. Bothmethods result in additional manufacturing costs and/or wasted mixedmaterial 12. However, method 70 as described below regulates systempressure Ps to the target pressure while system 10 is closed whileactively compensating for hysteresis within system 10 and pressureregulator 40. Additionally, method 70 can optionally regulate systempressure Ps to a target pressure that is offset to counteract theinitial pressure drop within system 10 when spray gun 14 is opened.

FIG. 2 is a flow chart showing method 70 of controlling system pressurePs within a closed system (i.e., system 10 between operatingconditions). Method 70 includes step 72 and the subsequent steps asdescribed below.

Step 72 includes selecting and sending a pressure set point and a flowrate set point to controller 50. The specific pressure and flow rate setpoints are determined based on the requirements of mixed material 12,for instance, as explained in the previously described example.

In step 74, controller 50 determines the state (e.g., closed or open) ofsystem 10. The controller can make this determination by receivingsignals that communicate the position of the trigger or solenoid valveof spray gun 14. If system 10 is closed, step 76 a is performed. Step 76a establishes a target pressure at spray gun 14 that is equal to thepressure set point plus a pressure offset. The pressure offset isselected to offset the effects of increasing or decreasing the pressureset point relative to the previously selected set point, as previouslydescribed above. Optionally, the pressure offset can also counteract theinitial pressure drop within system 10 when spray gun 14 is opened. Ifsystem 10 is open, step 76 b is performed. Because spray gun 14dispenses mixed material 12 when system 10 is open, offsetting thetarget pressure is not necessary. Thus, step 76 b establishes a targetpressure equal to the pressure set point.

After establishing a target pressure, step 78 involves calculating thepressure signal error. The pressure signal error is determined byreceiving signal 51 from pressure transducer 52 at controller 50 andcomparing signal 51 to the target pressure. The difference betweensignal 51 and the target pressure is the pressure signal error, which isstored over time in controller 50.

In step 80, the pressure signal error is used to update the PID loop.Proportional-integral-derivative loops or PID loops are known in theart. Updating the PID loop involves adding the current signal error to adata set of prior collected pressure signal error values. Next, theaccumulated pressure signal error values along with parameters inputtedinto the controller while tuning the controller initially are used tocreate a new pressure output signal C1. Output signal C1 is transmittedto control valve 42 in step 82.

In step 82, output signal C1 causes control valve 42 to increase ordecrease pilot pressure Pp thereby changing system pressure Ps usingpressure regulator 40. For example, if the pressure signal errorindicates that the pressure target is less than current system pressurePs, then controller 50 will transmit signal C1 commanding control valve42 to increase pilot pressure Pp. Conversely, if the error indicatesthat the target pressure is greater than current system pressure Ps,then controller 50 will transmit signal C2 commanding control valve 42to decrease pilot pressure Pp.

Following step 82 is step 84 in which controller 50 determines the stateof system 10 for a second time. The manner in which controller 50determines the state of system 10 is substantially similar to step 74.If system 10 is closed, steps 76 a, 78, 80 and 82 are repeated. Ifsystem 10 is open, controller 50 performs steps 86, 88, and 90.

Step 86 involves calculating the flow rate error within system 10.Controller 50 receives signals S2 and S3 from sensors 60 and 62 locatedon meters 24 and 28, respectively. The current flow rate R within system10 is equal to the flow rates R1 and R2 flowing through meters 24 and28, respectively. In other embodiments of system 10, a single meter(e.g., meter 24) can be used or additional meters (not shown) can beused depending on the number of components used to form mixed material12. In each case, flow rate R dispensed from spray gun 14 is equal tothe summation of each component flowing through one or more metersincluded in system 10. To determine the flow rate signal error,controller 50 compares the flow rate set point to the total flow rate Rof system 10. The flow rate signal error is the difference between theflow rate set point and flow rate R. Using the flow rate signal error,controller 50 updates a pressure-flow table in step 88 and determines anew pressure set point in step 90. The pressure-flow table is storedwithin controller 50 and relates system pressure Ps to flow rate R for aspecific mixed material 12. Following step 90, steps 74, 76 a or 76 b,78, 80 and 82 are repeated until the state of system 10 is open in step84.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. A method of varying a system pressure of a sprayer system used todischarge a fluid includes: sensing an open state or a closed state of asprayer system; and setting a first target pressure of the sprayersystem, wherein: if the sprayer system is in the open state, the firsttarget pressure equals a pressure set point of the sprayer systemcorresponding to a first flow rate of the fluid; and if the sprayersystem is in a closed state, the first target pressure equals asummation of the first pressure set point and a first pressure offset.2. The method of claim 1, and further comprising: selecting the firstpressure offset to counteract a system pressure change resulting fromthe absence of fluid flow within the sprayer system in the closed stateat the first target pressure.
 3. The method of claim 1, and furthercomprising: setting a second target pressure different from the firsttarget pressure, wherein: if the sprayer system is in an open state, thesecond target pressure equals a second pressure set point of the sprayersystem corresponding to a second flow rate of fluid that is differentthan the first flow rate; and if the sprayer system is in a closedstate, the second target pressure equals a summation of the secondpressure set point and a second pressure offset.
 4. The method of claim3, wherein the second pressure offset is greater than the first pressureoffset.
 5. The method of claim 3, wherein the second pressure offset isless than the first pressure offset.
 6. The method of claim 1, whereinthe first pressure set point corresponds to a system pressure at apressure regulator upstream from a spray nozzle from which fluiddischarges from the system in an open state and downstream from a fluidpump.
 7. The method of claim 3, and further comprising: selecting thesecond pressure offset to counteract a system pressure change resultingfrom the absence of fluid flow within the sprayer system in the closedstate and at the second target pressure, wherein the second pressureoffset is different than the first pressure offset.